1. Field of the Invention
The present invention generally relates to methods for using a pressure swing adsorption (PSA) zone in combination with a catalytic hydrocarbon conversion zone to improve the purity of a hydrogen-rich gas stream, to improve the recovery of liquefiable hydrocarbons from the hydrocarbon effluent of the catalytic hydrocarbon conversion zone, and to improve the recovery of liquefiable hydrocarbons from PSA tail gas.
2. Description of the Related Art
Various types of catalytic hydrocarbon conversion reaction systems have found widespread utilization throughout the petroleum and petrochemical industries for effecting the conversion of hydrocarbons to different products. The reactions employed in such systems are either exothermic or endothermic. Of more importance to the present invention, the reactions often result in either the net production of hydrogen or the net consumption of hydrogen. Such reaction systems, as applied to petroleum refining, have been employed to effect numerous hydrocarbon conversion reactions including those which predominate in catalytic reforming, ethylbenzene dehydrogenation to styrene, propane and butane dehydrogenation, etc.
Petroleum refineries and petrochemical complexes customarily comprise numerous reaction systems. Some systems within the refinery or petrochemical complex may result in the net production of hydrogen. Because hydrogen is relatively expensive, it has become the practice within the art of hydrocarbon conversion to supply hydrogen from reaction systems which result in the net production of hydrogen to reaction systems which are net consumers of hydrogen. Occasionally, the net hydrogen being passed to the net hydrogen-consuming reactions systems must be of high purity due to the reaction conditions and/or the catalyst employed in the systems. Such a situation may require treatment of the hydrogen from the net hydrogen-producing reaction systems to remove hydrogen sulfide, light hydrocarbons, etc. from the net hydrogen stream.
Alternatively, the hydrogen balance for the petroleum refinery or petrochemical complex may result in excess hydrogen, i.e., the net hydrogen-producing reaction systems produce more hydrogen than is necessary for the net hydrogen-consuming reaction systems. In such an event, the excess hydrogen may be sent to the petroleum refinery or petrochemical complex fuel system. However, because the excess hydrogen often has admixed therewith valuable components, such as C3+ hydrocarbons, it is frequently desirable to treat the excess hydrogen to recover these components prior to its passage to fuel.
Typical of the net hydrogen-producing hydrocarbon reaction systems are catalytic reforming, catalytic dehydrogenation of alkylaromatics and catalytic dehydrogenation of paraffins. Commonly employed net hydrogen-consuming reaction systems are hydrotreating, hydrocracking and catalytic hydrogenation. Of the above-mentioned net hydrogen-producing and consuming hydrocarbon reaction systems, catalytic reforming ranks as one of the most widely employed. By virtue of its wide application and its utilization as a primary source of hydrogen for the net hydrogen-consuming reactions systems, catalytic reforming has become well known in the art of hydrocarbon conversion reaction systems.
It is well known that high quality petroleum products in the gasoline boiling range including, for example, aromatic hydrocarbons such as benzene, toluene and the xylenes, are produced by the catalytic reforming process, such as a continuous catalyst regeneration (CCR) platforming process, wherein a naphtha fraction is passed to a reaction zone wherein it is contacted with a platinum-containing catalyst in the presence of hydrogen. Generally, the catalytic reforming reaction zone effluent, comprising gasoline boiling range hydrocarbons and hydrogen, is passed to a vapor-liquid equilibrium separation zone and is therein separated into a hydrogen-containing vapor phase and an unstabilized hydrocarbon liquid phase. A portion of the hydrogen-containing vapor phase may be recycled to the reaction zone. The remaining hydrogen-containing vapor phase is available for use either by the net hydrogen-consuming processes or as fuel for the petroleum refinery or petrochemical complex fuel system. While a considerable portion of the hydrogen-containing vapor phase is required for recycle purposes, a substantial net excess is available for the other uses.
Because the dehydrogenation of naphthenic hydrocarbons is one of the predominant reactions of the reforming process, substantial amounts of hydrogen are generated within the catalytic reforming reaction zone. Accordingly, a net excess of hydrogen is available for use as fuel or for use in a net hydrogen-consuming process such as the hydrotreating of sulfur-containing petroleum feedstocks. However, catalytic reforming also involves a hydrocracking function among the products of which are relatively low molecular weight hydrocarbons including methane, ethane, propane, butanes and the pentanes, substantial amounts of which appear in the hydrogen-containing vapor phase separated from the reforming reaction zone effluent. These normally gaseous hydrocarbons have the effect of lowering the hydrogen purity of the hydrogen-containing vapor phase to the extent that purification is often required before the hydrogen is suitable for other uses. Moreover, if the net excess hydrogen is intended for use as fuel in the refinery or petrochemical complex fuel system, it is frequently desirable to maximize the recovery of C3+hydrocarbons which are valuable as feedstock for other processes.
The pressure swing adsorption (PSA) process provides an efficient and economical means for separating a multi-component gas feedstream containing at least two gases having different adsorption characteristics. The more strongly adsorbable gas can be an impurity which is removed from the less strongly adsorbable gas which is taken off as product; or, the more strongly adsorbable gas can be the desired product, which is separated from the less strongly adsorbable gas. For example, it may be desired to remove carbon monoxide and light hydrocarbons from a hydrogen-containing feedstream to produce a purified, i.e., 99+%, hydrogen stream suitable for hydrocracking or other catalytic process where these impurities could adversely affect the catalyst or the reaction. On the other hand, it may be desired to recover more strongly adsorbable gases, such as ethylene, from a feedstream to produce an ethylene-rich product.
In pressure swing adsorption, a multi-component gas is typically fed to at least one of a plurality of adsorbent beds at an elevated pressure effective to adsorb at least one component, i.e., the adsorbate fraction, while at least one other component passes through, i.e., the non-adsorbed fraction. At a defined time, the feedstream to the adsorbent bed is terminated and the adsorbent bed is depressurized by one or more cocurrent depressurization steps wherein pressure is reduced to a defined level which permits the separated, less strongly adsorbed component or components remaining in the adsorption zone to be drawn off without significant concentration of the more strongly adsorbed components. The released gas typically is employed for pressure equalization and for subsequent purge steps. The bed is thereafter countercurrently depressurized and often purged to desorb the more selectively adsorbed component of the feedstream from the adsorbent and to remove such gas from the feed end of the bed prior to the repressurization thereof to the adsorption pressure.
Such PSA processing is disclosed in U.S. Pat. Nos. 3,430,418, 3,564,816, and 3,986,849, wherein cycles based on the use of multi-bed systems are described in detail. As is generally known and described in these patents, the contents of which are incorporated herein by reference as if set out in full, the PSA process is generally carried out in a sequential processing cycle that includes each bed of the PSA system.
Many processes for the purification of hydrogen-rich gas streams from the effluent of hydrocarbon conversion reaction zones are disclosed. U.S. Pat. No. 3,431,195 discloses a process wherein the hydrogen and hydrocarbon effluent of a catalytic reforming zone is first passed to a low pressure vapor-liquid equilibrium separation zone from which zone is derived a first hydrogen-containing vapor phase and a first unstabilized hydrocarbon liquid phase. The hydrogen-containing vapor phase is compressed and recontacted with at least a portion of the liquid phase and the resulting mixture is passed to a second high pressure vapor-liquid equilibrium separation zone. Because the second zone is maintained at a higher pressure, a new vapor liquid equilibrium is established resulting in a hydrogen-rich gas phase and a second unstabilized hydrocarbon liquid phase. A portion of the hydrogen-rich vapor phase is recycled back to the catalytic reforming reaction zone with the balance of the hydrogen-rich vapor phase being recovered as a hydrogen-rich gas stream relatively free of C3-C6 hydrocarbons.
U.S. Pat. No. 5,178,751 discloses a method for recovering high purity hydrogen gas and increasing the recovery of liquid hydrocarbon products from a hydrocarbon conversion zone effluent wherein the reaction zone effluent is first separated in a vapor-liquid equilibrium separation zone into a first hydrogen-containing vapor phase as a first liquid hydrocarbon phase. One portion of the first hydrogen-containing vapor phase is compressed and recycled back to the catalytic reaction zone. The balance of the hydrogen-containing vapor phase is cooled and recontacted with a portion of the first liquid hydrocarbon phase and passed to a second vapor-liquid separation zone to provide a second hydrogen-containing vapor phase and a second hydrocarbon phase. The second hydrogen-containing vapor phase is admixed with a portion of the first liquid hydrocarbon phase, refrigerated and passed to a third vapor-liquid separation zone to provide a high purity hydrogen stream and a third liquid hydrocarbon phase. The liquid hydrocarbon phases are collected and passed to fractionation for recovery of liquid hydrocarbon products.
Other references which disclose processes for improving the recovery of a hydrogen-rich gas stream reaction zone effluent comprising hydrogen and hydrocarbons from a hydrocarbon conversion zone include U.S. Pat. Nos. 4,568,451, 4,374,726, and 4,364,820.
U.S. Pat. No. 5,332,492 discloses a particular arrangement of a pressure swing adsorption zone, separators and refrigeration equipment that dramatically improves the purity of the hydrogen recovered and improves recovery of liquefiable hydrocarbons in such a system with only a relatively simple arrangement of components. However, tail gas derived from the PSA process in such an arrangement will still contain quantities of hydrocarbons with three or more carbon atoms (C3+) such as propane and isobutane.
Therefore, there is a need in the art for a process and apparatus that can be used to recover C3+ hydrocarbons from PSA tail gas as these C3+ hydrocarbons are valuable as liquefied petroleum gas (LPG) products. By recovering residual C3+ in the PSA tail gas, one could increase the yield of LPG and C5+ (5 or more carbon atoms) hydrocarbons recovered from a CCR platforming unit and improve the overall economies of the combined process.